ALD is a thin film growth method using alternating, self limiting reactions between gaseous precursors and a solid surface to deposit materials in an atomic layer-by-layer fashion. These attributes allow highly conformal and uniform films to be deposited on complex, three-dimensional substrates such as aerogels, powders, and anodic aluminum oxide (“AAO”) membranes useful in numerous applications. For instance, functionalization of a porous substrate by ALD allows for the synthesis of useful catalytic materials. It would be advantageous to construct catalytic structures with localized regions of different catalytic materials in a specific sequence along the internal walls of the nanopores of a substrate. Such structures could afford precise control over a multi-step catalytic process because the reacting feed stream would encounter the different catalytic sites in a predetermined order. It also might be useful in creating internal wiring structures for solar to fuels devices.
Heretofore, conventional ALD on porous substrates resulted in uniform infiltration of the deposited material into the entirety of the porous substrate, forming a uniform coating on all of the interior surfaces. Although a variety of techniques have been developed to allow patterning of ALD films using masking or passivating agents, these techniques are limited to planar substrates. For example, self assembled monolayers of long-chained, hydrophobic alkanes such as docosyltrichlorosilane (“DTS”) have been used as a masking agent for selective ALD of zinc oxide. However, such techniques are limited to depositing a mask pattern on a flat surface.
Thus, there is a need for methods capable of precisely applying various materials at localized regions within nano-porous materials. It is desirable to overcome limitations, such as those described above, to facilitate fabrication of structures having one or more coatings within nano-porous materials.